An expandable polystyrene resin particles is known well as an expandable thermoplastic resin particle, and an in-mold expanded mold article can be produced inexpensively and easily by using the expandable polystyrene-type resin particle. However, expanded mold articles are not sufficient in dimensional stability under a high-temperature environment for a long period of time upon use of relatively-high-temperature applications such as a thermal insulating material for pipes, a heat-insulating material for roofs, an automotive member, a thermal insulating material for solar systems or the like because the monomer that forms the polymer in the expanded mold article is styrene. The expansion-molded polystyrene article has thus a drawback that it is not suitable for those applications which require heat resistance. Further, even higher heat-resistant performance is required in these application fields in recent years.
In order to solve these problems, as the method for improving the heat resistance, Patent Document 1 proposes a method in which the content ratio among α-methylstyrene, acrylonitrile and styrene is defined. In this method, however, styrene remains in the monomer composition and therefore the heat resistance performance is not sufficient in the quality to meet the market requirement.
In Patent Documents 2 to 4, proposed are the methods in which α-methylstyrene, acrylonitrile, styrene and the like are copolymerized together and further the average chord length or the contents of an easily volatile foaming agent or a plasticizer are controlled to improve heat resistance performance. In these methods, expansion at up to 40 times can be achieved, and an expanded mold article can be produced with excellent surface characteristics. In this method, however, the heat-resistant performance is not sufficient due to the influence of styrene remaining in the monomer composition and thus there is still a room for improvement.